What are the issues that are driving this?
In the case of the Antioch Patriarchate, it is currently involved in a "jurisdictional dispute" with the Jesrusalem Patriarchate and therefore felt unable to participate in the Eucharist. As a result, it withdrew. A turf war by any other name.
As for the Bulgarians, they listed six objections. These included the absence from the agenda of the council topics of particular importance for Orthodox Christianity “that have contemporary relevance and require a timely Pan-Orthodox Council resolution”.
The Bulgarian Synod did not say what these topics were.
The Holy Synod said that the Autocephalous Orthodox Churches had already officially declared disagreements on some of the texts already approved for the council meeting.
It also objected to the rule that at the Pan-Orthodox Council, texts being discussed would not be subject to editing in the course of discussions.
Further, the Bulgarians objected that the proposed seating arrangements for the primates of the Orthodox Churches in the meeting room “violates the principle of equality of the primates of the Autocephalous Orthodox Churches”.
It also objected to the “inappropriate location of observers and guests of the Pan-Orthodox Council”.
Finally, the Holy Synod objected to the need to undertake “large and unjustified” expenses for the Bulgarian Orthodox Church to take part in the Pan-Orthodox Council.
I'd call that "trivial"
As for the Serbians, who knows? However, I suspect that both the Serbs and the Bulgarians have been put up to this by Moscow which doesn't really want the Council to go ahead and this is its way of flipping the bird to the Ecumenical Patriarchate in Constantinople.
“What are the issues that are driving this?”
This document is a start, but I’ve read a lot more sincere concerns beyond the ones mentioned here: